Enhanced fertilizer products with polymer adjuvants

ABSTRACT

Improved fertilizer mineral compositions are provided by coating a mineral, such as gypsum, a member of the Kieserite Group, potassium magnesium sulfate, elemental sulfur, and mixtures thereof, with low pH maleic-itaconic copolymers. The preferred copolymers are aqueous dispersions of acid or partial salt maleic-itaconic copolymers, and are applied by spraying or other means onto the surface of the mineral and allowed to dry. The copolymer coatings increase the solubility of sulfate and calcium or magnesium ions from the fertilizer minerals, allowing accelerated plant availability and uptake of such nutrients.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is broadly concerned with fertilizer compositionsand methods wherein the compositions include agriculturally valuableminerals, such as gypsum, members of the Kieserite Group, potassiummagnesium sulfate, and elemental sulfur, together with an amount of acopolymer adjuvant or additive serving to significantly increase theavailability of sulfate and other ions in soil, particularly during theearly stages after application of the fertilizer compositions. Moreparticularly, the invention is concerned with such fertilizercompositions and methods wherein the dried residue of an aqueous, verylow pH copolymer dispersion (e.g., either a true solution or a mixture)coats the minerals and wherein the copolymer includes respectivequantities of maleic and itaconic moieties.

2. Description of the Prior Art

Gypsum is a soft mineral compound formed of calcium sulfate (CaSO4),usually found in nature in the dihydrate form (CaSO4.2H2O). It iscolorless to white with a silky, pearly, or waxy luster, and commonlyhas various colored impurities. Gypsum occurs in nature as flattened andoften twinned crystals and transparent cleavable masses called selenite.It may also be granular or quite compact. Gypsum has a number ofcommercial uses, and is well known as a fertilizer and soil conditioner.In the late 18th and early 19th centuries, Nova Scotia gypsum, oftenreferred to as plaister, was a highly sought fertilizer for wheatfields. Gypsum is also used in ameliorating sodic soils. A significantadvantage of gypsum is that it is relatively low in cost, whilecontaining substantial quantities of nutrients; e.g., commercial gypsumgrades normally contain about 22% Ca and 17% S.

When applied to soils, gypsum supplies crop-available forms of calciumand sulfate ions. However, owing to the very limited solubility ofgypsum in water, positive fertilizing results or soil amendments mayrequire a two or three year program of applications. Hence, althoughgypsum fertilizer use is well-recognized, the slow action of gypsum doesnot provide immediate help, either in the form of plant nutrition orsoil remediation.

Kieserite is a form of magnesium sulfate (MgSO₄.H₂O) having a monocliniccrystalline system. There are a number of related minerals, known as theKieserite Group, which includes other hydrated forms of magnesiumsulfate, such as Epsomite (MgSO₄.7H₂O); the group also includes arelatively rare zinc-rich mineral Gunningite (Zn,Mn)(MgSO₄.H₂O).Comprehensive lists of Kieserite Group minerals may be found in Hammel(1939) Annales de Chimie, Paris: 11:247, and Palache, C., Berman, H., &Frondel, C. (1951), The System of Mineralogy of James Dwight Dana andEdward Salisbury Dana, Yale University 1837-1892, Volume II. John Wylieand Sons, Inc. New York, 7^(th) Edition, Revised and Enlarged, 1124 pp.477, both of the foregoing being expressly incorporated in theirentireties by reference herein. The Kieserite Group, because of thevaluable plant nutrients therein, can be used as plant fertilizers.

Elemental sulfur is also commonly applied to soils in order to supplysulfate ion. However, elemental sulfur is converted to sulfate veryslowly in normal soils, and thus the beneficial effects of sulfurapplications may take several growing seasons to appear.

There is accordingly a need in the art for enhanced forms of fertilizerminerals which accelerate the availability of sulfate and other ions(e.g., Ca and Mg) in the soil.

SUMMARY OF THE INVENTION

The present invention overcomes the problems outlined above and providesimproved mineral fertilizer compositions comprising respectivequantities of a mineral together with a copolymer in contact with themineral and containing individual quantities of maleic and itaconicmoieties. The minerals used in the invention are generally selected fromgypsum (either in the refined anhydrous or hydrated form), the KieseriteGroup, potassium magnesium sulfate, elemental sulfur, and mixturesthereof.

When gypsum is employed, it may be of any commercial grade suitable forfertilizer use, and may be naturally occurring or synethized as abyproduct. Advantageously, the gypsum is in the form of granules orpellets having a maximum dimension of up to about one inch, and morepreferably up to about one quarter inch. As used herein, “commercialgrade agricultural gypsum” refers to a gypsum product containing atleast about 80% by weight calcium sulfate (more preferably from about84-97% by weight), from about 19-27% by weight elemental calciumequivalent (more preferably from about 21-25% by weight), and from about15-23% by weight elemental sulfur equivalent (more preferably from about16-21% by weight). Such gypsum would also include impurities such ascalcium carbonate and other trace materials.

Any suitable Kieserite Group mineral can also be used, and especiallyKieserite itself. The only real criteria are commercial availability andcost. The same sizes of granules or pellets described with reference togypsum may also be used with the Kieserite Group minerals. In general,it is preferred that Kieserite be used having a minimum of about 14% byweight Mg and 20% by weight S therein.

Potassium magnesium sulfate, sometimes referred to as “KMag,” has theformula K2Mg(SO4)2 and analysis of 0-0-22. It contains about 22% K2O,11% Mg, and about 22% S. It is available in anhydrous and hydrated form,the latter usually a hexahydrate.

Elemental sulfur in granular or other forms can also be used in theinvention as a soil nutrient.

The preferred copolymer adjuvants of the invention are applied to orused with the mineral fertilizers as aqueous, low pH copolymerscontaining maleic and itaconic moieties, usually derived from thecorresponding acids or anhydrides. While other monomers may form apartof the maleic-itaconic copolymers, the itaconic and maleic monomersshould together form the preponderant fraction of the copolymers.Advantageously, other monomers should be present only in minor amountsof up to about 7% by weight, more preferably up to about 4% by weight,based upon the total weight of the copolymer taken as 100% by weight.Stated otherwise, the copolymers should comprise at least about 93% byweight, more preferably about 96% by weight, of a combination ofitaconic and maleic monomers. Most preferably, the copolymer consistsessentially of or is entirely made up of maleic and itaconic moieties.Ideally, the polymer fraction consists essentially of theitaconic/maleic copolymers, i.e., it is essentially free of other typesof monomers. The copolymer is preferably an aqueous solution ordispersion and is highly acidic. The pH usually ranges from about0.1-2.2, more preferably from about 0.1-2, and most preferably fromabout 0.2-0.8. Where partial salt forms of the copolymer are employed,the pH levels would be on the higher end of the foregoing ranges. Avariety of salt-forming cations may be used, but sodium and potassiumare preferred.

Generally, the fertilizer compositions include predominant amounts ofmineral fertilizers of from about 95-99.95% by weight, more preferablyfrom about 97-99.93% by weight, and most preferably from about 98-99.9%by weight, with the total weight of the composition taken as 100% byweight. Correspondingly, the copolymer fraction is the dried residue ofthe initially aqueous copolymers described above, and such residueshould be present at a level such that the plant availability of calciumand/or sulfate ion (in the case of gypsum or elemental sulfur) ormagnesium and/or sulfate ion (in the case of the Kieserite Groupminerals) is greater than the corresponding plant availability of theuntreated fertilizer minerals; more preferably, the plant availabilityof such ions in the case of the compositions of the invention is atleast about 15%, and most preferably at least about 30%, greater thanthat of the respective uncoated fertilizer minerals. In terms of weightamounts, the dried residue of the copolymers is generally from about0.05-5% by weight, more preferably from about 0.07-3% by weight, andmost preferably from about 0.1-2% by weight, with the total weight ofthe composition taken as 100% by weight.

The compositions may be simply prepared by spraying or otherwiseapplying the low pH aqueous copolymer to the mineral fertilizers,followed by drying to yield the dried residue of the copolymer on thesurfaces thereof. Such coated compositions are then used in the usualfashion by application to soils, with or without incorporation into thesoil. Although less desirable, it would also be possible to initiallyapply fertilizer minerals followed by an application of copolymer as afield dressing.

A particular advantage of the fertilizer compositions of the inventionis the enhanced availability of sulfate and other ions when applied tosoil. Further, the fertilizer compositions generate continuingquantities of these nutrients. This results from the fact that thehighly acidic copolymer reacts in the soil with calcium or magnesiumsulfate to yield sulfuric acid plus a partial calcium or magnesium saltof the copolymer formed in situ. The so-formed partial copolymer saltthen reacts in the soil to generate further quantities of the acid formof the copolymer and sulfates. Thus, a cyclic reaction is generated inthe soil, which provides sustaining quantities of plant nutrients. Sucha beneficial result arises from the use of the low pH copolymers, ascompared with copolymers of higher pH.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Maleic-itaconic copolymers are described in U.S. Pat. Nos. 6,515,090 and6,706,837, both fully and completely incorporated by reference herein,with special reference to the operative examples of the '837 patent. Ingeneral, the copolymers should desirably contain from about 10-90% byweight maleic moieties (more preferably from about 25-75% by weight),and correspondingly from about 90-10% by weight itaconic moieties (morepreferably from about 75-25% by weight). One particularly preferredcopolymer of this class is a 40% by weight solids aqueous copolymerdispersion of substantially equimolar amounts of itaconic and maleicanhydride moieties and having a pH of about 0.5.

The most preferred polymers of the invention are reaction products ofthe following maleic acid and itaconic monomers:

where X is individually and respectively taken from the group consistingof cations, preferably hydrogen, Na, K, and mixtures thereof, and them:n ratio ranges from about 99:1 to about 1:99. In order to obtain thedesirable low pH values for aqueous dispersions of the copolymer, thecopolymer may be used in acid form (i.e., all or essentially all Xmoieties are H), or partial salts wherein the preponderant amount of Xmoieties are H and some of the X moieties are Na, K, or mixturesthereof.

This reaction product of the above-described reactants has the generalformula:

where X is as defined above and p from about 10 to 500.

The following examples set forth preferred fertilizer compositions ofthe invention. It is to be understood, however, that these examples areprovided by way of illustration and nothing therein should be taken as alimitation upon the overall scope of the invention.

Example 1

In this example, the preferred copolymer of the invention was used tocoat gypsum, and the resultant improved solubility of the coated gypsumwas evaluated. The coating material was an acid form aqueous copolymer(pH 0.5) having approximately 40% by weight solids and made up ofsubstantially equimolar amounts of maleic and itaconic moieties. Thecopolymer was applied at two levels, namely 1 gallon per ton of gypsumand 1½ gallons per ton of gypsum.

Commercial grade agricultural gypsum pellets or granules were coatedwith the aqueous copolymer dispersion using a standard rotating cementmixer. The gypsum was first placed within the mixer, and thereafter thecopolymer was applied in sufficient quantities to substantiallycompletely coat all of the gypsum pellets. The rotation of the mixer wascontinued until the pellets were dry to touch. The dried coated pelletsincluded about 0.2% or 0.3% by weight dried copolymer residue thereon,based upon the total weight of the fertilizer compositions taken as 100%by weight.

Next, average weight of uniformly sized coated gypsum pellets wasdetermined. Triplicate samples were then prepared, each samplecontaining ten of the weighed pellets for the control (no copolymer),and coated pellets (0.2 and 0.3% by weight dried copolymer residue).Each sample was placed in a flask containing 5 ml of deionized water,and the respective sample sets were shaken for periods of five, ten, andtwenty minutes using a standard laboratory rotary shaker. Next, eachshaken sample was filtered to remove the pellets, and the liquidfiltrate sample was analyzed in a Varian ICP-MS (inductively coupledplasma-mass spectrometer) to determine the concentrations of sulfatesulfur and Ca therein. The pH of each liquid sample was also determined.

The following Table 1 sets forth the results of this study.

TABLE 1 Dried Polymer 5 Minute Extraction (Avg.) 10 Minute Extraction(Avg.) 20 Minute Extraction (Avg.) Concentration % S % Ca pH % S % Ca pH% S % Ca pH None 2.15 3.63 6.4 1.49 2.33 6.4 2.01 3.13 6.3 0.2% 2.033.14 5.9 1.89 2.99 6.0 2.58 3.99 6.1 0.3% 1.78 2.86 5.7 1.94 2.91 6.02.18 3.46 6.0 p > f 0.47 0.28 <0.01 0.19 0.23 0.08 0.16 0.34 0.10LSD_((0.10)) NS NS 0.2 0.47 NS 0.3 0.51 NS 0.2

Example 2

In this study, the same copolymer coated gypsum pellets described inExample 1 were used. However, in this example, three sets ofquadruplicate samples of the 0.2% by weight and 0.3% by weightcompositions, and three sets of quadruplicate uncoated control samples,were prepared. Each such sample contained approximately 1 gram of thecoated or uncoated pellets. These samples were each placed within 10 mlof deionized water and shaken using the laboratory shaker for periods of5, 10, and 20 minutes. Thereafter the samples were filtered and theconcentrations of Ca and sulfate sulfur in the filtered liquid weredetermined using the ICP-MS. pH values were also determined. Thefollowing Table 2 sets forth the results of this study.

TABLE 2 % by wt % by wt Sample/Shaker Time Ca Sulfate Sulfur pHControl/5 minutes 0.96 0.82 6.46 Control/5 minutes 0.96 0.82 6.62Control/5 minutes 0.8  0.70 6.74 Control/5 minutes 0.94 0.82 6.65 0.2%/5minutes 1.04 0.90 5.83 0.2%/5 minutes 0.94 0.80 5.75 0.2%/5 minutes 0.850.73 6.10 0.2%/5 minutes 1.21 1.02 5.83 0.3%/5 minutes 0.98 0.84 5.790.3%/5 minutes 0.99 0.87 5.76 0.3%/5 minutes 1.06 0.92 5.67 0.3%/5minutes 1.05 0.92 5.66 Control/10 minutes 0.90 0.81 6.65 Control/10minutes 0.86 0.77 6.72 Control/10 minutes 0.85 0.77 6.77 Control/10minutes 1.00 0.81 6.71 0.2%/10 minutes 1.17 1.01 6.03 0.2%/10 minutes0.90 0.80 6.16 0.2%/10 minutes 1.14 1.01 5.94 0.2%/10 minutes 0.93 0.845.94 0.3%/10 minutes 1.01 0.88 5.93 0.3%/10 minutes 1.12 1.00 5.760.3%/10 minutes 1.28 1.12 5.65 0.3%/10 minutes 1.25 1.11 5.71 Control/20minutes 0.93 0.86 6.75 Control/20 minutes 0.95 0.87 6.80 Control/20minutes 0.83 0.79 6.85 Control/20 minutes 0.85 0.80 6.88 0.2%/20 minutes0.95 0.89 6.34 0.2%/20 minutes 1.11 1.01 6.23 0.2%/20 minutes 1.17 1.056.11 0.2%/20 minutes 1.36 1.23 6.17 0.3%/20 minutes 0.92 0.85 6.030.3%/20 minutes 1.11 0.98 6.09 0.3%/20 minutes 1.05 0.95 6.15 0.3%/20minutes 0.93 0.85 5.97

The average sulfate sulfur and Ca values and statistical analyses were:

Control/5 minutes 0.79 0.2%/5 minutes 0.86 0.3%/5 minutes 0.89 % S p > f0.24 % S LSD_((0.10)) 0.10 % Ca p > f 0.29 % Ca LSD_((0.10)) 0.13Control/10 minutes 0.81 0.2%/10 minutes 0.92 0.3%/10 minutes 1.03 % Sp > f 0.07 % S LSD_((0.10)) 0.14 % Ca p > f 0.06 % Ca LSD_((0.10)) 0.17Control/20 minutes 0.83 0.2%/20 minutes 1.04 0.3%/20 minutes 0.91 % Sp > f 0.06 % S LSD_((0.10)) 0.14 % Ca p > f 0.07 % Ca LSD_((0.10)) 0.17These data are statistically significant, as noted above, anddemonstrate that the solubility of commercial grade granular or pelletedgypsum is enhanced by the copolymers of the invention. As such, thecompositions hereof provide better and accelerated plant availability ofsulfate and calcium. The quick-acting gypsum fertilizer composition madethrough use of the copolymer coatings provides an attractive Sfertilizer and a more available source of soluble Ca for soil amendment(e.g., in peanut production where supplemental Ca is sometimesimportant), or where high concentrations of Na producing undesirablesoil physical conditions may be replaced by the soluble Ca provided withthe present fertilizer composition.

Example 3

In this series of tests, the preferred polymer-coated gypsum fertilizerwas Example 1 was tested with bermudagrass to ascertain improvements inyield and quality of product.

Materials and Methods

Four repetitions of five plots each measuring 5′×20′ were randomlyassigned on an established Tifton 44 bermudagrass pasture in LafayetteCounty, Arkansas. The soil type, Severn silt loam, had a pH of 7.6. Theplots were on a deep, well drained, gently undulating soil located onthe flood plain along the Red River. Slopes were 0-3%. Soil tests weretaken before treatments were applied. Phosphorus (P) and potassium (K)were adequate at 75.4 ppm and 146.8 ppm, respectively.

Initial plot treatments were (1) a negative control (NC), no sulfuradded; (2) uncoated gypsum (CaSO₄) at a rate of 20 lbs S/acre; (3) 0.2%polymer coating on gypsum applied at a rate of 20 lbs S/acre; (4) 0.3%polymer coating on gypsum applied at a rate of 20 lbs S/acre and (5)ammonium sulfate ((NH₄)2SO₄) applied at a rate of 87 lbs/acre (21 lbsS/A). Ammonium nitrate (NH₄NO₃) was also applied to all plots, bothinitially and on each harvest date at a rate of 96 lbs/acre, except forthe ammonium sulfate plots. In the latter, ammonium nitrate was appliedat 50 lbs/acre due to the N contribution from the ammonium sulfate.

The plots were harvested at four week intervals using a Craftsman 6.75horsepower push mower with clipping bag to collect grass samples.Harvest swaths were 3″×22″×20′. After samples were collected, totalsample weights were recorded, a grab sample was taken and weighed forfresh weight and dry weight to determine dry matter content (DM) and tocompute yield per acre.

The plot samples were analyzed for feed quality. Specifically, thesamples were dried at 60° C. and ground with a Wiley Mill through a 2 mmstainless steel screen, then scanned using the FOSS NIR 5000 todetermine quality of the bermudagrass. Statistical analysis wasperformed using a randomized complete block design in SAS (8).

A second set of dry ground samples was analyzed for complete inorganiccontent. Using the dry weight and nutrient concentrations, nutrientuptake per acre was computed.

RESULTS AND DISCUSSION

These field studies indicate that polymer coating of gypsum was mosteffective in the first cutting following application. Polymer coatingsof 0.2 and 0.3% in the first cutting increased yield by approximately19.5% over the no-polymer gypsum when evaluated as a source of sulfurfor hybrid bermudagrass. There was no advantage for the higher polymerconcentration.

In the second cutting, the lower polymer coating did not increasebermudagrass yield over the uncoated control but the higher 0.3% coatingdid provide a 10% yield increase.

Understandably, the coated gypsum did not perform as well as ammoniumsulfate as a S source in the first cutting. However, by the secondcutting the polymer-coated gypsum was more effective than the highlysoluble ammonium sulfate suggesting luxury consumption of S by the cropin the first cutting when ammonium sulfate was the S source. On theother hand, S availability was extended by the slower solubility of thepolymer-coated gypsum.

These studies confirm that the polymer coating can and does enhance Savailability from granular gypsum, thereby increasing plant yields underS deficient soil conditions. The following Table 3 shows the yieldresults of this series of tests. The bermudagrass samples were alsoassayed for sulfur uptake, as set forth in Table 4 below.

TABLE 3 Cutting 1 Cutting 2 Total Treatment lb/A *% inc lb/A *% inc lb/ANo gypsum control 2465 — 1960 — 4424 Uncoated gypsum 2583 — 2087 — 4671Gypsum + 0.2% polymer 3089 19.6 2082 −0.2 5170 Gypsum + 0.3% polymer3088 19.5 2193 10.1 5232 Am. sulfate 3222 — 2043 — 5340 p > f 0.005 0.90— LSD_(.10) 340 NS  580 *% increase over uncoated gypsum

TABLE 4 Cutting 1 Cutting 2 Total S Uptake S Uptake S Uptake Treatment %S (lb/A) % S (lb/A) (lb/A) No gypsum 0.13 3.20 0.18 3.53 6.73 Uncoatedgypsum 0.15 3.88 0.21 4.38 8.26 Gypsum + 0.2% 0.12 3.71 0.19 3.96 7.67polymer Gypsum + 0.3% 0.14 4.29 0.24 5.20 9.49 polymer Am. sulfate 0.258.24 0.25 5.11 13.35 p > f 0.001 0.005 — LSD_(.10) 0.034 0.030 —

1. A fertilizer composition comprising respective quantities of afertilizer mineral selected from the group consisting of gypsum, one ormore members of the Kieserite Group, potassium magnesium sulfate,elemental sulfur, and mixtures thereof, and a copolymer in contact withsaid fertilizer mineral, said copolymer selected from the groupconsisting of acid or salt copolymers containing individual quantitiesof maleic and itaconic moieties, said copolymer being the dried residueof an aqueous copolymer dispersion having a pH of from about 0.1-2. 2.The fertilizer composition of claim 1, said copolymer comprising atleast about 93% by weight of itaconic and maleic moieties.
 3. Thefertilizer composition of claim 1, said copolymer being a copolymercontaining from about 10-90% by weight maleic moieties, and from about90-10% by weight itaconic moieties.
 4. The fertilizer composition ofclaim 1, said copolymer being present at a level of from about 0.05-5%by weight, based upon the total weight of the composition taken as 100%by weight.
 5. The fertilizer composition of claim 1, said copolymerbeing present at a level such that the plant availability of sulfateand/or calcium and/or magnesium ion is greater than the correspondingplant availability of the uncoated mineral fertilizer.
 6. The fertilizercomposition of claim 1, said mineral fertilizer being in the form ofcommercial grade agricultural gypsum.
 7. The fertilizer composition ofclaim 1, said copolymer being a partial salt of Na, K, and mixturesthereof.
 8. The fertilizer composition of claim 1, said copolymer beingessentially free of any moieties other than said maleic and itaconicmoieties.
 9. A method of fertilizing soil comprising the step ofapplying a fertilizer composition to soil, said composition comprisingrespective quantities of a fertilizer mineral selected from the groupconsisting of gypsum, one or more members of the Kieserite Group,potassium magnesium sulfate, elemental sulfur, and mixtures thereof, anda copolymer in contact with said fertilizer mineral, said copolymerselected from the group consisting of acid or salt copolymers containingindividual quantities of maleic and itaconic moieties, said copolymerbeing the dried residue of an aqueous copolymer dispersion having a pHof from about 0.1-2.
 10. The method of claim 9, said copolymercomprising at least about 93% by weight of itaconic and maleic moieties.11. The method of claim 9, said copolymer being a copolymer containingfrom about 10-90% by weight maleic moieties, and from about 90-10% byweight itaconic moieties.
 12. The method of claim 9, said copolymerbeing present at a level of from about 0.05-5% by weight, based upon thetotal weight of the composition taken as 100% by weight.
 13. The methodof claim 9, said copolymer being present at a level such that the plantavailability of sulfate and/or calcium and/or magnesium ion is greaterthan the corresponding plant availability of the uncoated mineralfertilizer.
 14. The method of claim 9, said copolymer being a partialsalt of Na, K, and mixtures thereof.
 15. The method of claim 9, saidcopolymer being essentially free of any moieties other than said maleicand itaconic moieties.
 16. The method of claim 9, said mineralcomprising agricultural gypsum.
 17. A method of preparing a fertilizercomposition including the steps of providing a quantity of a fertilizermineral selected from the group consisting of gypsum, one or moremembers of the Kieserite Group, potassium magnesium sulfate, elementalsulfur, and mixtures thereof, and at least partially coating saidfertilizer mineral with a copolymer selected from the group consistingof acid or salt copolymers containing individual quantities of maleicand itaconic moieties, said copolymer being in aqueous dispersion andhaving a pH of from about 0.1-2, and allowing said dispersion tosubstantially dry so that the dried residue of the dispersion is incontact with said fertilizer mineral.
 18. The method of claim 17, saidcopolymer having a pH of from about 0.2-0.8.
 19. The method of claim 17,said copolymer being a partial salt of Na, K, and mixtures thereof. 20.The method of claim 17, said copolymer being present at a level of fromabout 0.05-5% by weight, based upon the total weight of the compositiontaken as 100% by weight.
 21. The method of claim 17, said copolymerbeing present at a level such that the plant availability of sulfateand/or calcium and/or magnesium ion is greater than the correspondingplant availability of the uncoated mineral fertilizer.
 22. The method ofclaim 17, said mineral comprising agricultural gypsum.